Recently, direct injection gasoline engines have been shown to improve engine performance and to reduce transient air-fuel disturbances that may be caused by fuel adhering to the intake manifold and ports of an engine. However, at higher engine speeds and higher engine loads, particulates may form in engine exhaust. Under some conditions, formation of the particulates may be related to the short amount of time between when fuel is injected to the cylinder and when combustion is initiated by a spark plug. Specifically, there may be only a short opportunity for the injected fuel to completely vaporize and form a homogeneous mixture before combustion is initiated. If a homogeneous air-fuel mixture is not formed in the cylinder before combustion is initiated, pockets of stratification may form, and soot may be produced by combusting rich areas within the cylinder air-fuel mixture. Particulate filters have been proposed as one way to reduce emissions of soot.
The inventors herein have developed a method for regenerating a particulate filter without having to provide an external air supply. In particular, the inventors have developed a method for regenerating a particulate filter of a direct injection gasoline engine, comprising: increasing an overlap between an intake valve and an exhaust valve of a cylinder in response to an operating condition of a particulate filter.
By increasing overlap between an intake valve and an exhaust valve of a cylinder, air may pass from the intake system to the exhaust system so that excess oxygen is available to regenerate a particulate filter. And, since air passing through the cylinder does not affect the cylinder air-fuel mixture, the cylinder may be operated at stoichiometric or rich conditions so that less NOX is produced by the cylinder during regeneration of the particulate filter. In one embodiment, the boost pressure and cylinder fuel amount may be adjusted in response to a condition of a particulate filter so that the amount of excess oxygen in the exhaust gases may be matched to the amount of soot held by the particulate filter. In another embodiment, the boost pressure and cylinder fuel amount may be adjusted in response to a rate of oxidation of soot held by the particulate filter.
The present description may provide several advantages. Specifically, the approach may improve engine emissions by allowing a cylinder to operate at stoichiometric conditions while regenerating a particulate filter. Further, the amount of excess oxygen supplied to oxidize soot held by the particulate trap may be varied so that the soot is combusted efficiently. Further still, oxygen can be supplied to a particulate filter in the exhaust system without the need of an auxiliary air pump.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.